Wrought machinable tool steels



United States Patent 2,914,400 WROUGHT MACHINABLE TOOL STEELS George A. Roberts, Latrobe, Pa., assignor to Vanadium- Alloys Steel Company, Latrobe, Pa., a corporation of Pennsylvania No Drawing. Application April 8, 1954 Serial No. 421,949

11 Claims. (Cl. 75-123) This invention relates to a method for improving the machinability of wrought steel and to a new steel composition having improved properties. More particularly, the invention relates to the incorporation of lead sulfide in wrought steel to improve its machinability and to the resulting steel.

It has been known for many years that improvements in the machinability of steel can be made by adding a material which is more or less immiscible in the liquid state, and which generally causes a separation of a foreign particle within the structure of the steel in the solid state. For this purpose, the addition of sulfur has been most frequently employed to cause the formation of various non-metallic inclusions, principally manganese sulfide.

' Patented Nov. 24, 1959 It is an object of the invention to provide a method for producing a free machining steel by the addition of. a non-metallic component that is easily distributed throughout the steel.

Another object is to provide a method overcoming the disadvantages of prior methods, particularly that of segre- These inclusions are highly detrimental in certain instances, because of reducing the fatigue properties of the steel, or of reducing the transverse mechanical properties, or of causing imperfections to appear at the surface of parts made from the steel after machining.

Another common method involves the addition of metallic lead to the steel. The lead particles melt as they are introduced into the liquid steel, and by the use of very special techniques and great care, they can be caused to disperse throughout the metal and to solidify again as the metal freezes and cools. These individual particles, which are sometimes associated with normal inclusions in the steel, are the cause of the free machining characteristics imparted. When properly distributed they are less objectionable to the surface appearance, or the properties of the steel, than the sulfur inclusions. However, the introduction of lead in this form is fraught with many difiiculties. Almost invariably a large portion of the lead added sinks to the bottom of the ingot before solidification of the steel is complete, because of the great difference in the specific gravities of the two immiscible liquids involved, namely, lead and iron. This causes spitting out of lead from the bottomof the ingot on rolling or forging, and causes the loss of a large quantity of material through rejections in this portion of the ingot.

In addition, one of the great hazards encountered by the introduction of lead concerns the potential damage to the health of personnel. At the time of introduction, the lead volatilizes partially, and the fumes are known to be cumulative if inhaled and to cause a severe health hazard. This hazard extends also to subsequent forging and rolling operations on the leaded steels, in which the steel must, of necessity, be heated to near or above the melting point of the finely divided lead particles contained therein.

Other addition agents are known to produce similar effects in improving the machinability. The addition of gation of the additive, which is of widespread application and can be carried out simply, reliably and at low cost.

An additional object is to provide a new steel composition having good machinability without undesired properties.

These and other objects and advantages of the inventionwill appear on consideration of the specification.

It has now been discovered in accordance with the invention that a very advantageous free machining steel is produced by incorporating lead sulfide in the steel. A very important characteristic of lead sulfide is that its specific gravity at or about the temperature of molten steel is approximately the same as that of the steel, so that it is readily dispersed throughout the steel bath and throughout the resulting ingots. Lead sulfide also has a high melting point and a high dissociation temperature so' that it is not nearly so dangerous to handle as lead metal.

The lead sulfide is preferably added to the molten steel bath just prior to pouring in the molds for the formation of ingots. The resulting solidified ingots contain the generally immiscible solid lead sulfide uniformly distributed throughout. The machinability of the steel is greatly improved and little or no difiiculty is encountered in subsequent rolling or forming operations. Dispersed lead sulfide may be provided in the steel in the molds in other ways.

Finely divided lead sulfide, or galena, is incorporated in the steel in an amount sufiicient to improve its machinability. The amount will vary with the extent of the improvement desired. In general, lead sulfide may be added in an amount of from 0.1% to 1% by weight of the steel to provide improved machinability without undesirably affecting the product. An average addition of about 0.3% is generally preferred and effective. The

' galena available contains slightly more sulfur than corzirconium sulfide and chromium sulfide to stainless steels responds to the ratio of lead and sulfur in the monosulfide. The foregoing amounts refer to such a material.

The invention is widely applied to wrought steels. Thus, plain steels, alloy treated steels, and alloy steels including simple alloy or ternary steels, quaternary steels and complex alloy steels may be treated in the invention. Low, medium and high carbon steels containing from about 0.3% to about 2.3% carbon are suitable. Steels intended for various uses, including tool, production and stainless steels, and manufactured by the several processes, e.g., crucible, bessemer, open-hearth, duplex and electric furnace steels are included in the invention.

The invention is particularly useful in the preparation of the lower carbon steels without major alloying elements, especially the wrought tool and special alloy steels, as described in Part III, Chapter IX, pages 1015- 1057 of The Making, Shaping and Treating of Steel, Camp and Francis, 5th ed., 3rd Imp. (Carnegie-Illinois Steel Corp., 1940). The invention is applicable to the carbon and modified carbon tool steels, especially those containing about 0.6% to 1.5% of carbon, the low alloy tool steels, the medium alloy and high-chromium tool steels, and the high-speed steels.

The following examples are furnished to assist in providing a complete understanding of the invention, but it is to be understood that the invention is not limited thereto nor to the specific compositions, proportions and procedures set forth therein, which are given only for purposes of illustration.

Example 1 About 0.3% of the lead sulfide of commerce is added to the molten steel bath in the production of carbon tool steels containing iron and from 0.6% to 1.5% of carbon. Shortly after the addition, the bath is poured into molds for the formation of ingots.

The solidified steel ingots are subsequently processed in rolling or forging operations without difiiculty. The machinability of the wrought steel products is greatly improved and their properties are otherwise very good. Rejections are low, not increasing over the rejections of products prepared without an additive.

Example 2 About 0.3% of lead sulfide is added to the molten steel as in Example 1, in the production of low alloy tool steels of the manganese oil-hardening type containing 0.9% carbon, 1.2% to 1.6% manganese and approximately 0.2% vanadium. The steel may also contain approximately 0.5% chromium and 0.5% tungsten. Again, the machinability of the wrought steel products is greatly improved without substantial undesired efiects.

Example 3 About 0.3% of lead sulfide is added to the molten steel bath in the production of low alloy tool steel of the chromium air-hardening type containing 1% of carbon, 5% of chromium, and 1% of molybdenum. Again, improved machinability without serious disadvantages is obtained.

Example 4 High carbon-high chromium steels of the six general types are treated in the same manner by the addition of about 0.3% of lead sulfide, the steels containing about 1% to 2.25% of carbon, about 10% to 14% of chromium and about 1% of molybdenum. They may also contain up to 2% each of silicon, tungsten, vanadium, and cobalt. Another type contains 3.5% of cobalt. Machinabilityis improved without harmful effects.

Example 5 Similarly, about 0.3% of lead sulfide is incorporated in tungsten hot work die steels containing 0.3% to 0.5% of carbon, 8% to 18% of tungsten, 2% to 4% of chromium, and 0.25% to 1% of vanadium. Machinability is improved without harmful effects.

Example 6 Chromium hot work steels containing about 0.35% of carbon, 5% of chromium, 1% to 1.5 of molybdenum, 0.2% to 1% of vanadium, and, optionally, 1% of tungsten are treated with lead sulfide in the same manner. Again, machinability is improved without disadvantageous results.

Example 7 The various types of high-speed steel are similarly treated with lead sulfide. The steels contain about 0.7% to 1.5% of carbon, to 18% of tungsten, 4% of chromium, 1% to of vanadium, 0% to 8% of molybdenum, and 0% to 8% of cobalt. The addition of lead monosulfide provides considerable improvement in machinability, and production of satisfactory products is very good.

Wrought steel as used herein means any article of steel formed by forging or hot rolling, or by hot working in any way.

What is claimed is:

1. A high speed steel containing about 0.1% to 1% by weight of lead sulfide.

2. A high speed steel containing about 0.1% to 1% by weight of lead sulfide, about 07-15% of carbon, up to about 18% of tungsten, about 4% of chromium, about 1-5% of vanadium, up to about 8% of molybdenum, up to about 8% of cobalt, and the balance substantially iron.

3. A high speed steel containing about 0.1% to 1% by weight of finely divided lead sulfide dispersed throughout the steel, about 0.7-1.5% of carbon, up to about 18% of tungsten, about 4% of chromium, about 15% of vanadium, up to about 8% of molybdenum, up to about 8% of cobalt, and the balance substantially iron.

4. A low alloy tool steel of the manganese oil-hardening type containing about 0.1% to 1% by weight of lead sulfide, about 0.9% of carbon, about 1.21.6% of manganese, about 0.2% of vanadium, and the balance substantially iron.

5. A low alloy tool steel of the manganese oil-hardening type containing about 0.1% to 1% by weight of lead sulfide, about 0.9% of carbon, about 1.21.6% of manganese, about 0.2% of vanadium, about 0.5% of chromium, about 0.5 of tungsten, and the balance substantially iron.

6. A low alloy tool steel of the chromium air-hardening type containing about 0.1% to 1% by weight of lead sulfide, about 1% of carbon, about 5% of chromium, about 1% of molybdenum, and the balance substantially Iron.

7. A high carbon-high chromium steel containing about 0.1% to 1% by weight of lead sulfide, about l-2.25% of carbon, about 10-14% of chromium, about 1% of molybdenum, up to about 2% of silicon, up to about 2% of tungsten, up to about 2% of vanadium, up to about 2% of cobalt, and the balance substantially iron.

8. A high carbon-high chromium steel containing about 0.1% to 1% by weight of lead sulfide, about 1-2.25% of carbon, about 10-14% of chromium, about 1% of molybdenum, up to about 2% of silicon, up to about 2% of tungsten, up to about 2% of vanadium, about 3.5% of cobalt, and the balance substantially iron.

9. A tungsten hot work die steel containing about 0.1% to 1% by weight of lead sulfide, about 0.30.5% of carbon, about 8-18% of tungsten, about 2-4% of chromium, about 0.251% of vanadium, and the balance substantially iron.

10. A chromium hot work steel containing about 0.1% to 1% by weight of lead sulfide, about 0.35% of carbon, about 5% of chromium, about 1l.5% of molybdenum, about 0.21% of vanadium, and the balance substantially iron.

11. A chromium hot work steel containing about 0.1% to 1% by weight of lead sulfide, about 0.35% of carbon, about 5% of chromium, about 1-1.5% of molybdenum, about 0.2-1% of vanadium, about 1% of tungsten, and the balance substantially iron.

References Cited in the file of this patent UNITED STATES PATENTS 2,182,758 Harder Dec. 5, 1939 FOREIGN PATENTS 709,828 France May 26, 1931 OTHER REFERENCES The Selection and Hardening of Tool Steels, pages 3 to 8 inclusive, edited by Seabright, published in 1950 by the McGraw-Hill Book Company. 

1. A HIGH SPEED STEEL CONTAINING ABOUT 0.1% TO 1% BY WEIGHT OF LEAD SULFIDE. 